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The introduction of elastomeric behavior into polycarboranylenesiloxane SiB-1 polymers from the closo-carborane C2B5H7.

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JOURNAL OF APPLIED POLYMER SCIENCE VOL. 14, PP. 2525-2536 (1970)
The Introduction of Elastomeric Behavior into
Polycarboranylenesiloxane SiB-1 Polymers from the
CZoso-carborane C,B5H,
R. E. NESTING, K. F. JACKSON, E. B. KLUSMANN, and F. J.
GERHART, Chemical Systems Incorporated, Santa Ana, California 92705
Synopsis
The copolymerization of a small amount of the larger (C~BS
and C~BIO)
carboranes with
the smaller ClBs carborane resulted in the format.ionof an elastomeric SiB-1 carboranesiloxane, a polymer hitherto prepared only in crystalline form. Although the elastomeric
behavior of the uncured SiB-1 copolymer decreasedwith time, curing stabilizedthe rubber
by inhibiting crystallization.
INTRODUCTION
The utilization of carboranesiloxane polymers (Fig. 1) as elastomers has
been under intensive investigation for the last five
The first
carborane to be evaluated for this application was m-carborane, i.e., 1,7dicarba-closo-dodecarborane-12(1,7-C2B10H12). This largest of the carboranes (number of boron atoms x = 10) was incorporated into a siliconelike structure by preparing the bischlorodimethylsilyl and bismethoxydimethylsilyl derivatives and condensing the two in the presence of a ferric
chloride catalyst to yield a hard crystalline SiB-1 polymer (Fig. la). However, to obtain elastomeric properties it was found necessary to copolymerize the bismethoxydimethylsilyl m-carborane monomer with either
dichlorodimethylsilane to yield a SIB-2 polymer (Fig. lb) or dichlorotetramethyldisiloxane to yield a SiB-3 polymer (Fig. lc) (where 2 and 3 refer to
the numbers of oxygen atoms separating the carborane unit,s). Although
the thermal stability of the elastomeric SiB-2 and SiB-3 polymers based on
1,7-C2B10&2 was superior to that of unmodified methylsilicones, it was,
nevertheless, inferior to that of the corresponding SiB-1 polymers. The
lower thermal stability of the SiB-2 and SiB-3 relative to the SiB-1 polymers
is presumed to be due to the -0-Si-0bonds in the polymer backbone
which in effect converts them from modified carborane into modified silicone
polymers. The desirability of retaining the “short” siloxane moiety to
maximize thermal stability while, if possible, also retaining the elastomeric
property was suggested to one of our associates4several years ago.
Two years ago, synthetic routes to the lower carboranes (x < 10 in Fig. 1)
were developed to the point at which sufficient quantities became available
2525
@ 1970 by John Wiley & Sons, Inc.
KESTING, JACKSON, KLUSMANN, AND GERHART
x-SiB-2
x-SiB-3
x x
CH3
CH3
f
y i n branchmodified x-SiB-1
-4.5
Fig. 1. Carboranesiloxanepolymers.
for the inception of a polymerization p r ~ g r a m . ~
Such a program was
deemed desirable for several reasons: The properties of the polymers from
the smaller carboranes were unknown; the size of the smaller carboranes
made their inclusion into an elastomeric backbone seem sterically favorable;
and their small size suggested certain routes to the production of elastomeric SiB-1 polymers.
The present paper is the first in a series dealing with the preparation of
carboranesiloxane polymers from the smaller carboranes. It is concerned
with the synthesis and certain aspects of the microcrystalline structure of
both unmodified C2B5-SiB-1polymers and of CzB5-SiB-1polymers whose
crystallinity has been disrupted, and elastomeric behavior induced, by
copolymerization with monomers containing the larger (CZBs, Fig. Id, and
C2Blo,Fig. le) carborane moieties.
EXPERIMENTAL
2,4-Dicarba-cZoso-heptaborane-7(I). A mixture of the smaller, i.e.,
Ba-Bs, closo-carborane prepared according to the procedure of Ditter and
co-workers5was separated on a gas chromatograph (Hewlett Packard Model
776 equipped with four columns of 4 f t X 2.5 in. O.D. packed with ApiezonL on Chromosorb-P). To CzB5H7(>99% pure, containing <1% B5H9) was
ELASTOMERIC BEHAVIOR OF SiB-1
2527
added sufficient anhydrous diethyl ether to make a 2.OM solution. Sodium
hydride was then added in 100% excess to remove the last vestige of B5Hs,
and the solution was stirred for 2 hr before being vacuum transferred into a
storage bulb.
2,4-Dilithio-2,4-dicarba-cZoso-heptaborane-7
(11). The entire preparation was carried out in a drybox. To 35.1 g (0.412 mole) of CZB5H5 (2.0
M ) in ether at -5 f 2°C was added 120 ml of anhydrous hexane. n-Butyllithium, 1.2 moles (1.6M in hexane), was then added dropwise with stirring
over a 2- to 3-hr period. The salt (11) was precipitated by the addition of
400 ml of hexane to yield 68.0 g of pale-yellow powder, the etherate of 11.
2,4-Bis(chlorodimethylsilyl)-2,4-diearba-cZoso-heptaborane-7
(111). To
a three-necked 3-liter flask equipped with a stirrer, thermometer, nitrogen
inlet, and addition funnel were added 520 ml of (CH3)2SiC12 (dist. 3X)
and 1liter of anhydrous ether. The addition funnel contained a slurry of
178 g of I1 etherate in 800 ml of hexane. Addition of the slurry occurred
over a period of 2 hr while the flask was held at 0°C. The solution was
filtered and distilled under vacuum through a 48-in.-column packed with
glass helices; 110.35 g of I11 was collected at 64"-66"/0.9 mm (32.3%
yield, based on C2B5H,starting material).
2,4-Bis(methoxydimethyldyl) 2,4-dicarba closo heptaborane 7 (VI).
To a 500-ml flask filled with a stirrer and attached to a vacuum line were
added 100 g (0.369 mole) of I11 and 75 g (1.173 moles) of anhydrous methanol. After stirring at room temperature for 2.5 hr, methanol and HC1
were evaporated, and the procedure was repeated to produce a quantitative
yield of IV.
The Poly-C&H5-Carboranylenesiloxane Analog of SiB-1 (V). To a
reactor containing 9.515 g (0.0351 mole) of I11 and 9.20 g (0.0351 mole) of
I V was added 0.23 g (2 mole-% based upon total moles of I11 and IV) of
anhydrous FeC13.1-3 After evacuation at room temperature the solution
was slowly raised to 185°C. After a total reaction time of 3 hr, the solution was cooled and an additional 0.23 g of FeC13 was introduced. The
reaction was continued at 185°C in vacuo until the evolution of methyl
chloride was complete. The raw polymer was dissolved in hot xylene,
filtered, precipitated with methanol, washed with aqueous acetone, redissolved in xylene, and reprecipitated with methanol to yield 14.8 g
(97.7%) of V, a hard wax
= 12,500).
Dimethoxy(1-vinyl-o-carborane-2-y1)methylsilane
(VI). To 29.7 g (0.10
mole) of dichloro(l-vinyl-o-carborane-2-yl)methylsilane, prepared according to the procedure of Heying et a1.,6 was added an excess of methanol as
in the synthesis of IV above. The procedure was repeated three times to
yield VI, a white solid, in quantitative yield.
1,lO-Bis(methoxydimethylsily1)-1,lO-dicarba-doso-decaborane-10
(VII).
This compound prepared in the same manner as I V above but from 1,sbis(chlorodimethylsilyl)-cZoso-1,10-dicarbadecaborane-l0 obtained through
the courtesy of Professor F. Hawthorne and Mr. P. Garrett of UCLA.
Future publications describing carboranesiloxane polymers in which the
-
(nn
-
-
-
2528
KESTING, JACKSON, KLUSMANN, AND GERHART
C2Bs moiety is the principal component will be issued jointly with Professor
Hawthorne and Mr. Garrett.
CH,--CH(C2BloHll)-Modified Poly-C2B5H5-Carbornylenesiloxane
halogs of SiB-1 (VIII). To 50 mole-% of I11 and to 45, 40,and 35 mole-%
of IV were added 5, 10, and 15-mole-%, respectively, of VI and 2 mole-%
of FeC13. The reaction was carried out as for V above.
CzB&Xs-ModifiedPoly-C2B5H5-carboranylenesiloxane
Analogs of S i - 1
(IX). To 50 mole-% of I11 and 45 and 40 mole-% of IV were added 5 and
10 mole-%, respectively, of VII and 2 mole-% FeC13. The reactioh was
carried out as for VIII above.
Differential scanning calorimetry (DSC) measurements were made on a
du Pont Model 600 instrument at a scanning rate of 15"C/min. Infrared
spectrograms of xylene-cast films were obtained on a Perkin Elmer Model 21
infrared spectrophotometer.
Samples of C2B5-SiB-1modified with lO-mole-% (8.23 wt-%) of C2B8 were
compounded as follows: 100 parts resin; 10 parts SiOz (Quso F-20,
Philadelphia Quartz Company) ; and 5 parts Fe203 (Mapico Red-297,
Columbia Carbon Company). The mixture was blended on a hot tworoller mill and divided into two portions, to one of which was added 0.5 part
of 2,4-dichlorobenzoyl peroxide (K and K Laboratories). The sample
containing the peroxide was cured at 260°C for 26 hr. The rate of crystallization for both elastomers was estimated by measuring sample hardness (Shore A scale) with a durometer (PTC Instruments).
RESULTS AND DISCUSSION
Synthesis
The preparation of I1 (repeated over 20 times during the past two years)
proceeded readily at -5"C, in contrast to the recent experience of Grimed
who prepared I1 at the higher temperature of 25°C. Unlike the dilithio
salt of m-carborane (x = lo), I1 was only slightly soluble in ether. It did,
however, form an etherate which could be obtained in the form of a dry
powder by the addition of hexane. Compound I1 was added to dimethyldichlorosilane as a slurry in hexane in order to form III.8 The reverse
addition was not considered desirable because various secondary reactions
could occur unless dimethyldichlorosilanewas at all times present in excess.
Although I11 was initially purified on a gas chromatograph, a product of
equivalent purity was subsequently obtained by vacuum distillation. The
conversion of I11 to IV proceeded uneventfully. The polymerization reactions proceeded smoothly but not rapidly. This may have been due to
steric hindrance and/or competition between condensation and substitution
reactions. The fact that all of the SiB-1 polymers prepared were completely
soluble in hot xylene indicated that only a minimal amount, if any, of
undesired crosslirikingreactions occurred.
ELASTOMERIC BEHAVIOR OF SiB-1
2529
Characteristics of SiB-1 CarboranesiloxanePolymers
Thermal Stability
The substantial thermal stability of the polydimethyl- and polydiphenylsiloxanes can be attributed to the stability of the Si-0-Si
bond. However, thermal depolymerization by catalyst residues introduced during
polymer preparation, by metal compounds which act as Lewis acids, and by
water vapor, does occur. The incorporation of electron-deficient moieties
such as the carboranes (B)6+tends to oppose depolymerization by strengthening the siloxane bond via increasing dr-prr bond contributions :9
Because the extent of this bond-strengthening inductive effect will be
proportional to the concentration of carborane in the polymer, SiB-2 and
SiB-3 varieties exhibit lower thermal stability than SiB-1 types.
Crystalline versus Elastomeric Behavior
Two principal factors contribute to elastomeric behavior: (1) a coiled
molecular configurationwhich can be uncoiled by the application of suitable
stresses; and (2) low cohesive energy between segments on the same or
different polymer chains, i.e., weak intra- and intermolecular forces.
The cohesive energy in the polydimethylsiloxane elastomers is of an extremely low order,IOa condition which is attributable to a fairly homogeneous distribution of electrons along the entire molecule. However, the
introduction of electropositive (carborane) sites along a polymer segment
increases the latter's affinity for electronegative (oxygen) sites along another
polymer segment, so that the cohesive energy density of the carboranesiloxanes is higher than that of the unmodified siloxanes. Higher cohesive
energy, then, results in an increased tendency to crystallize, a situation
which is exacerbated when steric regularity also tends to favor crystallinity.
It is apparent that steric regularity is at a maximum in the SiB-1 carboranesiloxanes (Fig. la) and decreases with increasing concentration of
-Si-(CH3kOgroups. Thus the elastomeric behavior of the SiB-2
(Fig. lb) and SiB-3 (Fig. lc) polymers is superior to that of SiB-1 polymers
for both steric and electrostatic reasons. I n the case of the CzBlo carboranesiloxanes, a compromise was eventually reached between thermal
stability on the one hand and acceptable elastomeric behavior on the other
by the development of the SiB-2 and, more recently, the SiB-3 polymers.
Further developmental work on the polymer with the highest thermal
stability, CZBlo-SiB-1, has apparently been abandoned because of its high
crystallinity and consequent poor elastomeric properties.
The availability of the smaller carboranes appeared to offer a solution or
at least an alternative to this dilemma for two reasons: (1) the presence of
the smaller units enhanced the possibility of achieving the tightly coiled
KESTING, JACKSON, KLUSMANN, AND GERHART
2 530
Care A: y s x
Crystallization strongly
inhibited
Care 8: y c x
Crystallization weakly
inhibited
Fig. 2. Schematic representation of crystallite disruption in 2-SiB-1 carboranesiloxane
polymers by the inclusion of an occasional y-carborane where (A) y > 2; (B) y < 2.
configuration characteristic of elastomers; (2) the structural regularity, and
hence tendency to crystallize, of smaller units is more easily disrupted by
the insertion of an occasionallarge moiety than vice versa (Fig. 2).
The lesser bulk of the smaller carboranes was not sufficient per se to
induce elastomeric behavior into a C2Bs-SiB-1polymer, and the product
obtained was a hard wax whose Si-0-Si
band was characterized by an
infrared doublet with maxima at 9.05 and 9.45 microns (Fig. 3a). This
spectral feature was similar to that observed for the C&-Sib-1"
but
was unlike the singlet observed for Cil3~s-SiB-2.'~This suggested that
the SiB-1 doublet may be the result of restricted mobility of the Si-0-Si
bond owing to electrostatic interactions between the carborane moieties and
oxygen atoms in contiguous segments which resulted in crystallite formation.
TABLE I
DSC Study of the Disruption of (CZHs)-SiB-l Crystallinity by the
Inclusion of CzBs and CzHlo Cornonomers
Comonomer
None
CeB8
C2Ba
C2Bio
CzBio
CzBio
.
Comonomer content
mol-%
wt-%
Crystallinity
index AN*
0
5
10
5
10
15
0
5.50
8.28
5.28
10.23
15.83
0.795
0.0535
0.0437
0.01995
0.00536
1 .o
Glass
Melting
temp, T,, "C temp T,, "C
70
54
58, 45
59
60.5
61
4h
- 27b
- 50
-59
- 54
- 52
a A N = Area under DSC curves for samples of equal weight, normalized with respect
to the unmodified CzB,Sib-l.
b These values are not well defined (see Figs. 4 and 5 ) .
ELASTOR‘lERIC BEHAVIOR O F SIB-1
2531
,
7
8
9
10
11
WAVELENGTH (MICRONS)
Fig. 3. Infrared, spectra of C&-SiB-l’s samples (a) and (b) containing 0 and 5 mol-%
C~BS,
respectively.
At this juncture it was decided to determine whether the introduction of
occasional larger carborane moieties would disrupt crystallinity SUfEiciently
so as to result in an elastomeric SiB-1.
The feasibility of this approach
was suggested by previous work which showed that the stiffening temperature (a measure of crystallization tendency) of polydimethylsiloxanes was
decreased from -40°C to a minimum of -112°C by the introduction of 7.5
mol-yo of phenyl groups.12 This study also suggested the molar concentration range over which any such effect might manifest itself most strongly,
viz., between 5 and 15 mole-Y0. Toward this end two series of polymers
were fabricated: (1) that in which the comonomer was VI; and (2) that in
which the comonomer was VII.
Fortunately, the introduction of between 5 and 15 mole-% of VI or VII
did indeed have the desired effect of disrupting crystallinity (Table I, Figs.
4 and 5 ) . The raw polymers were highly elastomeric and soluble in hot
xylene (no crosslinking). The Si-0-Si
band of a C213sSiB-1 containing 5 mole-% of C2Bs
was a singlet (Fig. 3b). This is believed to be indicative of less restricted motion and hence more amorphous character than the
unmodified SiB-1.
The DSC study yielded qualitative, and even quan-
KESTING, JACKSON, KLUSMANN, AND GERHART
2532
I
-1-
I
t
-80
I
I
-60
I
t
-40
I
t
I
-10
1
0
'
1
+I0
'
1
+40
'
+b0I
"
'
too
Figure 4. DSC tracings for C&-modifled C2B5-SiB-1elastomers.
titative, information as to the extent of crystallite disruption by the insertion of an occasional large carborane unit (Table I, Figs. 4 and 5). A crystallinity index A , was defined for which the sample exhibiting the highest
extent of crystallinity, viz., the unmodified SiB-1, was arbitrarily assigned a value 1.0. The area under the DSC curves of the various samples
was then integrated and normalized relative to the unmodified SiB-1 after
making suitable corrections for minor variations in sample size. Figures 4
and 5 are representative of the raw DSC (uncorrected for sample size)
tracings.
Several important characteristics are apparent :
1. The complexity of the unmodified is much greater than that of the
modified SiB-1 polymers. Five separate DSC peaks, two of which are
most prominent, are discernible in the case of the former, whereas the latter
are characterized by at most two separate melting domains. These domains are believed to correspond to variations in crystallite size and/or
order. (Small and/or less perfect crystallites melt at lower temperatures
than large and/or more ordered structures.)
2. A N decreases with increasing concentration of the order-disrupting
larger carboranes. This fact constitutes strong evidence for the validity of
I
+lo0
ELASTOMERIC BEHAVIOR OF SiB-1
2533
8
f
10
I
I
-100
(msbndii)
I
I
-80
I
I
-bO
,
I
-40
I
I
-10
I
I
0
I
I
+10
I
I
+40
I
+b0I
Fig. 5. DSC tracings for CzBtwmodified CZBt,-SiB-l elastomers.
the assumption that the presence of occasional large units can successfully
disrupt crystallinity.
3. The values of T, and T, decrease with increasing concentration of
CZBs (Fig. 4 and Table I). However, the T, values for the unmodified
SiB-1 and the sample containing 5 mole-% of CZB, are at best barely
discernible and may, therefore, be erroneous. Decreasing values of T, and
T, are further indications of decreasing crystallite size and/or structural
order.
4. The values of T, and T, increase slightly with increasing concentration of the vinyl-C3Blomoiety (Fig. 5, Table I). This somewhat unexpected
result is not indicative of increased crystallinity ( A N decreases even more
dramatically than for equivalent CZBS incorporation), but instead is indicative of the presence of the vinyl groups, a fraction of which have very
probably participated in crosslinking the polymer. An increase in T, and
T, with increasing crosslink density is not unusual because of the inhibiting
effect of crosslinking upon segmental mobility.
The C2Bs-and vinyl-C2Blo-modified C3&3B-1 polymers are, of course,
not strictly comparable. The C2Bs-modifiedpolymers are true SiB-1
polymers in every respect because the 1,10-CzBs-carborane is incorpo-
2534
KESTING, JACKSON, KLUSMANN, AND GERHART
rated into the polymer backbone in the usual manner (Fig. Id), while
the ~inyl-C~B~0-1
polymers, on the other hand, are not only found on side
branches but also contain the vinyl grouping (Fig. le). These effects account for most of the more efficient crystallinity disruption of the C2Blo
modifier. Five mole-% of vinyl-C&O reduces the crystallinity to approximately one twentieth of the unmodified C2B5-SiB-1, a decrease
which requires 10 mole-% of the c2B8 to equal (Table I). Had both polymers of the “y in chain” type (Fig. Id), a much smaller difference between
the two would have been expected. Nevertheless, even in such a case the
C2B10modifier should be slightly more efficient than the C2B8, both because
of the greater size of the former and because the carbon atoms are in the
ortho position in C2B10and in the para position in CzBB. The para position
possesses greater structural regularity than the ortho position and consequently may be expected to exert a somewhat lesser order-disrupting influence.
The modified SiB-1 polymers are highly elastomeric materials when
freshly prepared, and their DSC tracings are indicative of a complete absence of crystallinity (Fig. 4c). On standing, however, crystallization does
occur (Fig. 4b), and decreasing elasticity and increasing hardness with time
are clearly apparent. This tendency to crystallize with time is attributable
to the persistence of the previously mentioned electrostatic interactions that
occur in spite of steric hindrance to the formation of virtual crosslinks. It
was felt, however, that the introduction of covalent crosslinks into the
polymer might provide a practical means for inhibiting the inter- and intramolecular realignment which ultimately results in crystallization.,
The experiment which was devised to test this hypothesis involved the
use of a peroxide catalyst in the test sample to effect crosslinking and unequivocally demonstrated the feasibility of this approach (Fig. 6).
Two blanks were employed: a highly crystalline unmodified SiB-1 and
one containing 10 mole-% of c38 in chain. The rate of crystallization was
conveniently followed by measuring the shore A hardness (the most convenient measure of crystallinity available) versus time at 23” f 1°C. The
most important results of this study are: (1) crystallization can be inhibited by crosslinking; (2) the hardness of the cured specimen increased
very slightly over the course of 16 hr and not at all thereafter; (3) the hardness of the uncured specimen, on the other hand, increased very rapidly and
exceeded that of the cured specimen after 5 hr; and (4) the hardness of the
uncured sample continued to increase over the course of several days,
asymptotically approaching a value at infinite time somewhat below that of
the unmodified SiB-1.
I n summary, the feasibility both of producing elastomeric C2B5SiB-1
polymers by the disruption of crystallization by the inclusion of occasional
larger carborane moieties and of maintaining the elastomeric behavior so
induced by the introduction of covalent crosslinks has been demonstrated.
Additional work is, however, necessary to determine the thermal stability
of the polymers so produced and to increase molecular weight and to deter-
(hours)
I
50
I
60
I
70
Fig. 6. Shore A hardness versus time for CZB5-SiB-1 carboranesiloxane polymers: (A)unmodified CZB&B-l;
( 0 ) CzBs-modified SiB-1 (cured).
Time
I
40
1
30
I
20
I
10
C2B8-modified SiB-1 (cured)
0
(uncured)
C2B8-modified
SiB-1
unmodified C2B5-SiB-1
0
4
( 0 ) CzBe-modified SiB-1
I
90
I
ao
I
(uncured);
00
cn
to
w
w
2536
KESTING, JACKSON, KLUSMANN, AND GERHART
mine optimum modifier concentrations, compounding formulations, and
curing conditions.
A subsequent report on the CZBsanalogs of the C2Blo-SiB-2 polymers is
currently in preparation. A report on the CzBeanalogs of the C2Blo-SiB-2
polymers will also be issued in the near future in collaboration with Professor M. F. Hawthorne and Mr. P. M. Garrett of UCLA.
The authors gratefully acknowledge the support of this study by the Office of Naval
Research. They would also like to express their gratitude to their colleagues Drs. R. E.
Williams, J. F. Ditter, H. D. Fischer of West Coast Technical Service, San Gabriel,
California, and Professor M. F. Hawthorne of the University of California at Los Angeles
for their helpful critical discussions, and to Dr. H. V. Seklemian, who participated in the
initial phases of the polymerization program.
References
1. S. Papetti, B. Schaeffer, A. Gray, and T. Heying, J . Polum. Sci. A-1, 14, 1623
(1966).
2. H. Schroeder, 0. Schaffing, T. Larcher, F. Frulla, and T. Heying, Rubber Chem.
Tech., 39,1184 (1966).
3. H. Schroeder, Rubber Age, 101,58 (1969).
4. H. Fox, Personal Communication to R. E. Williams.
5. J. Ditter, J. Oakes, E. Klusmann, and R. Williams, J. Inorg. Chem., 9, 889
(1970).
6 . T. Heying et al., Inorg. Chem., 2,1089 (1963).
7. R. Grimes, Paper Presented a t ONR Boron Conference, Stamford, Conn., Sept.
1969.
8. M. F. Hawthorne, Personal Communication.
9. W. Noll, Chemistry and Technology of Silicones, Academic Press, New York, 1968.
10. F. Billmeyer, Textbook of Polymer Science, Wiley, New York, 1965.
11. R. E. Williams and R. E. Kesting, Paper Presented at ONR Boron Conference,
Stamford, Conn., Sept 1969.
12. K. Polmanteer and M. Hunter, J. Appl. Polym. Sci., 1 , 3 (1959).
Received May 4, 1970.
Revised July 2, 1970.
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